Run 2 "instances" of blender at the same time?
You can run as many instances as you want off the same install. Just run the .exe again. Note that this may get more complicated if you are launching through a command prompt. I often run multiple instances, especially when doing physics simulations.

Note that when GPU rendering, it tends to cause slowdown/lag on your main screen. This is because the GPU resources are going to rendering. If you have multiple graphics cards it isn't as bad, but you can still get screens slowing down even if they are on a card that isn't rendering.